1. Field of the Invention
The invention is concerned with, generally, the domain of restorative and prosthetic dentistry, specifically, "fixed partial dentures," more commonly known as "fixed dental bridgework." The traditional or "indirect" method of producing fixed dental bridgework typically involves the extra-oral, or outside the patient's mouth, use of molds, metal frameworks, and pressurized ovens to produce bridgework. The "direct" method of producing fixed dental bridgework is done in situ, intraorally, entirely within the patient's mouth using particle-filled dental restorative resin (called "dental composite" or simply "composite"), and plain dental restorative resin, without reduction of the abutment teeth or the use of cement or luting agents.
2. Description of Related Art
Fixed dental bridgework has traditionally involved the following process: (1) during the first office visit by the patient, the dentist surgically reduces the anchor or "abutment" teeth on either side of a space (edentulous area) in a dental arch to be spanned by the bridgework; (2) the dentist makes an impression of the reduced abutment teeth and edentulous area; (3) the impression is sent to a laboratory for construction of a model to which the bridgework is conformed during fabrication; (4) metal "pontic" castings, a metal framework that holds the pontics (artificial teeth), and attachment wings are fabricated; (5) in a special high temperature oven, porcelain may then be fused to the pontic forms, and to the metal attachment wings on each end of the metal bridge framework if desired, depending on the original bridgework design; (6) the bridgework is sent from the laboratory to the dentist, (7) during a second office visit by the patient, the dentist inserts and adjusts the bridgework in the patient's mouth; and (8) the dentist "permanently" cements the attachment wings or crowns on the ends of the bridgework to abutment teeth to fix the bridgework in place. Examples lo of the type of bridgework described above are shown in U.S. Pat. No. 5,194,001 to Salvo and in U.S. Pat. No. 5,000,687 to Yarovesky, et al.
A poor fit between the traditional bridgework and the abutment teeth cannot be discovered until the finished bridgework is inserted into the patient's mouth; a poor fit sometimes develops after a period of wear. To cure a poor fit, the bridgework must be removed from the patient's mouth, modified, then reattached. Sometimes, several iterations of attachment, removal, modification, reattachment are necessary, each requiring an office visit by the patient. Attachment wings or crowns run the range of mechanical and/or adhesive devices, such as screws, foils, films, screens, mastics, hooks, etc. Frequently, the reduction and/or process of attachment (especially the use of screws) injures the abutment teeth and can lead to caries, abscesses, and/or tooth death. Removal of the bridgework after cementation of the attachment wings to the abutment teeth sometimes injures the abutment teeth, or even requires their removal.
For decades there has been a quest for a more efficient, effective, and non-invasive means of replacing missing teeth in a fixed manner. Extraordinary efforts have been devoted to trying to devise methods that do not require cutting or otherwise mutilating the abutment teeth. Two of the most common methods devised to avoid reducing the abutment teeth are well known as the "Maryland bridge" and the "Rochette bridge." The Maryland bridge and the Rochette bridge are constructed with a metal framework of nickel-chromium and beryllium in a laboratory, etched with an acid medium or sandblasted on the tissue side surface of the attachment hooks, and then cemented to the natural abutment teeth with a polymer luting agent. Due to the inflexibility of the metal frame and the weak bond of the polymer to metal and polymer to teeth, the attachment hooks can separate from the abutment teeth. There are additional serious disadvantages with the Maryland and Rochette bridges, as well as with other indirect bridgework. In the area of aesthetics, the underlying metal may "shine-through" the pontic surface, disrupting the color, hue, value and shade of the replacement tooth. The high-fusing-porcelain can rapidly abrade natural teeth opposite the bridge. The metallic content of the metal bridges sometimes precipitates allergic or even less understood impairment of the patient's health. There is a growing and real concern for the quality and quantity of metal used in dentistry and the deleterious effects on the bio-environment of the oral cavity. All three kinds of metals utilized in the previously mentioned bridges are bio-toxic to some degree. Nickel is known for its allergenic capacity and is an experimental carcinogen and equivocal tumor former. Chromium is a suspected carcinogen and an equivocal tumor producer. Beryllium is an equivocal neoplastic producer concerned with pulmonary problems that produce tumors and is an experimental carcinogen.
In additional to undesirable health side effects, laboratory fabricated porcelain/metal bridges have structural and aesthetic deficiencies. Structurally, the metal can fracture, the porcelain can fracture, and/or the porcelain/metal fused interface can separate. All of these structural failures require removal of the bridge for repair. Aesthetically, after cementation of the bridge, changes over time in pontic color or shade versus natural teeth, or bridgework fit, require removal of the bridge. All these deficiencies of indirect, laboratory fabricated porcelain/metal bridges are difficult, if not impossible, to resolve. The chronic failure of the metal-polymer-tooth bond, and the other deficiencies noted above, have prompted research into other bridgework materials, namely those in the porcelain, ceramic, and composite groups.
These other attempts to eliminate or reduce the deficiencies noted above all rely, however, on the "indirect method," that is, fabrication of bridgework in a dental laboratory or operatory. To improve the strength of the bridgework to abutment tooth bond, methyl-acrylate polymers doped with more durable filler particles, collectively known as "composites," were introduced. Attempts have also been made to affix a natural tooth or a fabricated pontic to abutment teeth utilizing webs of materials such as metal rods, carbon fiber rods, screens, films, and foils made of various materials.
U.S. Pat. No. 5,171,147, to Burgess, discloses a dental bridge prepared using the indirect method in which the bridgework pontic and attachment wings are fabricated as an integral unit out of heat-polymerized composite. Unlike traditional bridgework, the Burgess bridge does not use a metal framework, metal pontics, or fused porcelain, but like traditional bridgework, it uses the indirect method, including a pressurized oven. For installation in a patient's dental arch, the Burgess bridge requires reduction of the abutment teeth, reduction of the attachment wings to conform them with the attachment surfaces on the abutment teeth, and the use of cements or other bonding agent to fix the Burgess bridge in place. Structurally, the Burgess bridge "product" comprises not only a single, composite pontic with composite attachment wings, but the cement or other bonding agent necessary to give the bridge utility in situ.
Installing a fixed bridge without cement is contrary to the teaching of the prior art, even that of Burgess. Burgess does not disclose or claim a bridge that is bonded without a cement, or a bridge of more than one pontic, and the shown structure of the Burgess bridge has attachment contours that are useless without reduction of the abutment teeth and/or attachment wings and cementation. The Burgess bridge "product" is a two-element combination: pontic plus cement. Every claim in the Burgess patent recites, directly or indirectly, the limitation of cementing the bridge to the abutment teeth. An inventive step would be a composite bridge of one or more pontics that does not use cement or a luting agent, but is an integral bridge in situ, not a combination of "pontic plus cement." Moreover, the Burgess bridge has an implied limitation in the same way that a threaded bolt has an implied limitation. A threaded bolt requires a threaded nut or other threaded channel for utility. The Burgess bridge requires reduction of the abutment teeth for utility. All embodiments of the Burgess bridge are disclosed as requiring a reduction of the abutment teeth in the same way that a threaded bolt requires a threaded channel for utility. Fixed bridgework that in all, or virtually all, cases does not require reducing the abutment teeth is contrary to the teaching of the prior art.
Burgess describes and claims an "indirect" product that uses a bonding or luting agent different, or differently polymerized, from the pontic compound, which introduces two chemical interfaces, composite to cement, and cement to tooth. An inventive step would be bridge created in situ that does not use a different bonding or luting agent and has only one chemical interface, composite to tooth. A single compound, single chemical interface, completed product is a structurally different product from a two compound, two chemical interface, completed product. The Burgess bridge lacks utility without cement.
To use a different structural analysis, the Burgess bridge is made in a generic form that requires further adaptation for end use; it is a "workpiece" or "blank" lacking conforming attachment surfaces. The structure and form of the Burgess bridge permit it to be mass produced. In its end-product form, the Burgess bridge fits nobody without structural modifications to the cured composite and to the abutment teeth.
The indirect method has proven to be lengthy and complicated. Approximately ten laboratory steps are needed in the simplest traditional bridge construction, and with these steps come costs. Some methods are even more complex; for instance, U.S. Pat. No. 5,000,687 to Yarovesky et al. discloses an indirect method involving about 14 or 15 separate process steps. Furthermore, many indirect methods require the abutment teeth to be surgically reduced in some form; for example, the process disclosed in the patent to Yarovesky et alii requires cutting and contouring of the lingual surfaces of the abutment teeth prior to bridge installation. U.S. Pat. No. 5,120,224 to Golub, seeking to eliminate or minimize the need for abutment tooth reduction, discloses a bridge structure wherein a thin fabric laminate may be internally sandwiched within the pontic/bridge. The fabric extends outwardly from the pontic/bridge for placement on abutment teeth and for bonding thereto by an cement or luting agent. As stated in the Golub patent, however, the disclosed structure is intended to function only as a temporary or provisional bridge. Moreover, it has been found that the placement of an inappropriate fabric or screen within composite material, particularly within the attachment wings, may weaken, rather than strengthen, the bridge framework. All laboratory-based indirect methods are therefore relatively costly, time consuming, and ineffective.
U.S. Pat. No. 4,172,323 to Orlowski discloses a method for securing a previously-made pontic or a fixed bridge, wherein a thin film or screen is applied to the appropriate surfaces on the abutment teeth. Cement or a luting agent is applied to the screens and to the attachment wings of the pontic/bridge being installed, whereafter the pontic/bridge is held in place while the cement or luting agent cures. Orlowski, however, discloses that small undercuts are made in the enamel of the abutment teeth contact areas, so as to increase the area available for bonding and resistance to shear forces. Despite that attempt to increase contact areas, it has been found that these areas are still limited such that weakness of joints results. Consequently, the securement structure disclosed by Orlowski has been found to be temporary or at best provisional, i.e., lasting considerably less than five years.
Where tooth or dental implant extraction is required before insertion of fixed dental bridgework, regardless of whether the bridgework has been constructed directly or indirectly, the traditional approach requires two to four months healing of the extraction site (alveolar socket) prior to bridge installation. The fabrication and installation of a fixed dental bridge during the same office visit as extraction of teeth or implants from the area to be bridged has heretofore been regarded as impossible.
Accordingly, there is a need in the art for a method of directly producing a permanent dental bridge that (i) eliminates invasive tooth preparation steps, (ii) can be done during the same office visit as tooth or dental implant extraction, (iii) can bridge an edentulous area of one or more alveoli, and (iv) eliminates the need for separate cementation of the bridge to the abutment teeth.